This invention relates to a supply circuit for assuring the supply of voltage to an electronic load. More particularly, the invention relates to such a supply circuit for protecting data in the memory of a microprocessor in which the load is connected, via a series-connected rectifier, to a supply voltage and, via the series-connected drain-source path of a field-effect transistor, to an auxiliary supply voltage. The electronic load is connected in shunt to both supplies, one of its terminals being connected to the source electrode of the transistor.
A circuit arrangement of this general type, described in "Electronics," January 1981, pp. 129-132, contains a self-blocking field-effect transistor which is controlled by a comparator. A portion of the supply voltage is connected to the inverting input of the comparator; the non-inverting input is connected to an auxiliary voltage supply in the form of a battery. In this circuit, the connection is established every time a change of the supply voltage is detected by a processor. The comparator can then make a voltage comparison between the applied portion of the voltage from the supply voltage source and the voltage from the auxiliary voltage supply. If the voltages are equal, a switching signal is given to the gate electrode of the self-blocking field-effect transistor, the transistor is switched into conduction, and the electronic load is connected to the auxiliary voltage supply.
In another known circuit arrangement (EP-A-0 019 222), an electronic load is connected to a supply voltage via a first, self-blocking, field-effect transistor. An auxiliary voltage supply is connected in series with a diode which is polarized to become conducting if the level of the supply voltage drops. The auxiliary voltage supply, however, is only connected to the load, via a TTL logic circuit, when a transistor has been cut off. The logic circuit thereupon switches another self-blocking, field-effect transistor into conduction and closes the circuit from the auxiliary voltage supply to the electronic load.
The above-described circuit arrangements are relatively elaborate. It is therefore an object of the present invention to provide a circuit for assuring the supply of voltage to an electronic load which provides interruption-free switching of the load from a main voltage supply to an auxiliary voltage supply and which requires very little circuitry, being therefore very reliable.